This invention relates to toners for developing electrostatic latent images in electrophotography, electrostatic recording and electrostatic printing.
Stable copied images of high quality are obtained in electrophotography by visualizing directly or by inversely developing electrostatic latent images, wherein a cascade developing method (U.S. Pat. No.2,297,691, U.S. Pat. No. 2,618,552), a magnetic brush developing method (U.S. Pat. No. 2,832,311) using a developer composed of toner and carrier, a touch-down developing method (U.S. Pat. No. 412,931) or a non-magnetic one component developing method (U.S. Pat. No. 3,731,146) using a developer only composed of toner is used.
As for a toner suitable for those developing methods, a dye as a charge controlling agent, a pigment as a coloring agent and a wax as a peeling agent and the like are mixed with a thermosetting resin and the mixture is kneaded, pulverized and classified to prepare toner particles of mean particle size of 4 to 25 .mu.m. Inorganic fine particles such as silica, titanium oxide or aluminum oxide are usually added to endow the toner with fluidity and to improve cleaning properties.
However, some kinds of the inorganic fine particles, for example, titanium oxide have large primary particle size of up to 50 m.mu.. When such a titanium oxide of large particle size is used, there arise problems such as poor fluidity, low amount of initial charge due to decreased contact probability with carrier particles, so that copied images have many fogs and are poor in fine texture.
In the case of silica, silica is electrically charged to so high level that toner is also charged to high level. The concentration of copied images is lowered. Silica particles added to the toner usually have small particle size and, when they are used by being mixed with the carrier, there arises another problem that fluidity of the toner decreases because silica particles are buried into the toner surface. Silica tends to absorb moisture on its surface and is not good in moisture resistivity. Though a technique to apply hydrophobic treatment on the surface of silica particles has been proposed to solve the problem, further improvement in environmental resistivity has been desired yet.